Strips or bands for specific uses are provided in the literature and in industry, each strip or band consisting of filamentary rovings made of synthetic or natural material, which are connected to one another either by means of a binding material, whatever the degree of impregnation of the said rovings, or by means of woven, braided, lapped or twisted textile threads.
The technique for the first category has the disadvantage of requiring a large proportion of material in order to ensure that the rovings are bound to one another. At the present time, the proportion of material is of the order of 50% by volume, even when the fibres within the rovings are impregnated only partially or not at all. This results in a limited performance in terms of maximum permissible resistance per unit of cross-section. This is especially the case with regard to profiles made of the composite material FRP (Fibre Reinforced Plastic), such as a composite consisting of glass fibre or aramide fibre and thermosetting or thermoplastic resin, and as regards strips consisting of a lap of juxtaposed parallel rovings (for example, made of aramide) which are contained in a thermoplastic mass, such as polyamide, the said mass being longitudinally and transversely continuous.
Other disadvantages described below have been encountered in the use of this technique, depending on the type of binding material. When the material is deformable, such as a thermoplastic, for example a polyamide, it does not ensure that the radial position of the rovings or their position in relation to one another is maintained sufficiently firmly when the strip is subjected to high traction, such as when it is wound around a duct. It has been found, in fact, that the tensioned filaments, while bearing on a curved support, tend to approach the supporting surface and to push the lower filaments radially and laterally in spite of the binding material. This results in the material experiencing a creeping effect under the influence of contact pressure exerted by the filaments. By contrast, when the material is not deformable, such as a thermosetting resin, for example epoxy resin, the strip is rigid and is therefore unsuitable for being wound around a duct.
With regard to the technique of the second category, there have been improvements in assembling longitudinal rovings more efficiently. Thus, in weaving, it has been proposed to add to or substitute for the weft threads threads which comprise heat-meltable material, the heat-sealing property of this material being revealed after weaving. The function of the heat-meltable material is to ensure the stability of the rovings relative to one another after the fabric has been cut, as described in the FR 2,577,946.
The FR 2,677,047 is also known, this describing a textile reinforcement in the form of a plane band intended for the production of complex laminated structures. The reinforcement comprises spaced longitudinal threads forming a unidirectional structure and a flattened tubular braiding forming a bi-directional structure which surrounds the longitudinal threads. It likewise comprises threads comprising heat-meltable material for binding the longitudinal threads in the manner of a loose woven weft. After thermal treatment, the heat-meltable material ensures that the longitudinal threads are held relative to one another and are connected to the tubular braiding.
The products described in the above patents are intermediate reinforcing products involved in the composition of a complex laminated structure, for example tubes made of the composite material FRP. They are put to use by being embedded in a thermosetting resin and forming the reinforcing element of the latter after the resin has been cured. As such, they must have bare warp or braiding threads in order to allow good bonding of the resin serving as a matrix for the laminated article.
It was found that assembling the rovings by means of the textile technique has several disadvantages. First of all, the type of assembly is adversely affected by a phenomenon of a shortfall in the resistance of the rovings. The shortfall expresses, as a percentage, a loss of strength of the rovings, when they are assembled, in relation to the strength of the roving taken individually. The shortfall is a parameter which increases with the number of rovings to be assembled and which depends on the assembly technique used. It is equivalent to a loss of overall efficiency of the strip which may exceed 30 to 40%, depending on the types of assembly. As well as this, there is an additional loss of efficiency when the strip is subjected to traction while bearing on a curved surface. In this case, the rovings are not all subjected to stress together to their maximum resistance (unequal tension). This loss can be compensated only by adding new rovings, thus entailing financial expense and even more unfavourable efficiency.
The above-described strips or bands have been proposed for reinforcing ducts instead of metallic reinforcements on account of their performances and lightness. The strips or bands are used in the duct in two main ways:
by circular braiding around a tubular support involved in the formation of the duct, PA1 by spiral winding in a plurality of superposed laps around the same tubular support, the said strips being crossed from one lap to the other (so-called crossed laps). PA1 by increasing the number of superposed braidings or windings in the wall of the duct, PA1 by carrying out a preassembly of the rovings in sufficient numbers, before using them as a reinforcing element in the manufacture of the duct. PA1 at least one layer of longitudinal filamentary rovings, PA1 means for assembling the said filamentary rovings, the said assembly means comprising at least one binding material, characterized in that the assembly means comprise retaining means which are based on transverse elements and which are shaped so as to surround and clamp the said layer, the longitudinal filamentary rovings being agglomerated with one another in the form of a relatively flat bundle with a substantially rectangular cross-section, each filamentary roving having a specific average compactness of at least 60%. PA1 a surface coating of at least part of the filamentary rovings with a binding material is carried out; PA1 the longitudinal filamentary rovings of the bundle are held tensioned under a controlled load while the retaining means are wound around the said rovings, and PA1 the bundle thus assembled is compressed while the binding material is activated.
If appropriate, these rovings may be coated in the material, and the degree of coating of the filaments forming the rovings may vary, depending on the uses.
At the present time, there is a limitation to the diameter of the duct when the rovings are arranged by braiding. In fact, beyond a diameter of 40 mm, the means used for braiding assume considerable proportions on account of the number of bobbins and their dimensions.
There is another limitation as regards the performance of the ducts in terms of mechanical resistance under traction and under pressure. In fact, in order to obtain better performances, the tendency is to increase the number of rovings in the duct. This may typically be carried out by two means:
Experience has shown that the first means is unsatisfactory. It was found that, beyond two braidings and two pairs of crossed windings, the additional layers afford only a moderate contribution to the resistance of the duct.
As regards the second means, the various disadvantages recalled above in respect of the various techniques for assembling the rovings were found.
Another phenomenon adversely affecting textile assembly is the rotation of the ducts. In fact, it was found that ducts reinforced by being wound with conventional braided strips experience a rotational effect on their end when they are put under internal pressure and/or high tension. Such a phenomenon damages the ducts or makes it impossible to use them.
Damage was also found inside the strip or on its surface as a result of a phenomenon of abrasion which occurs when the duct is subjected dynamically to very high tensile, torsional or bending stresses. Such stresses arise, in particular, on ducts of the flexible hose type, when they undergo frequently repeated handling and laying operations or when they are used as a dynamic riser for the working of marine oil fields.
The disadvantages described above relate at least partially to elongate articles, such as hooping bands or strips, straps, flat ropes, belts and conveyer bands, when these are subjected, in particular, to tensile stresses while bearing on and around a curved surface.
The invention is therefore used preferably amongst the abovementioned articles or their equivalents.